1. Field of the Invention
The present invention relates to a start up circuit. More particularly, the present invention relates to a high voltage start up circuit for power converters.
2. Description of Related Art
FIG. 1 shows a circuit diagram of a traditional start up circuit, in which a voltage VD is required to provide a supply voltage to a control circuit 10 of a power converter. When the power converter is switched on, the voltage VD is supplied from a voltage source VIN through a transistor 11. A drain terminal and a source terminal of the transistor 11 are coupled to the voltage source VIN and the control circuit 10 respectively. When the control circuit 10 starts to operate, a further supply voltage is provided by a transformer winding 16 through a diode 17 and a capacitor 18. After that, the transistor 11 will be turned off to save the power consumption by a transistor 12. The transformer winding 16 is coupled to a ground and a terminal of the diode 17. The capacitor 18 is coupled between another terminal of the diode 17 and the ground. The capacitor 18 is further coupled to the control circuit 10. A resistor 15 is coupled from the drain terminal of the transistor 11 to a gate terminal of the transistor 11. The resistor 15 is used to provide a bias voltage to turn on the transistor 11.
A drain terminal and a source terminal of the transistor 12 are coupled to the gate terminal of the transistor 11 and the ground respectively. A gate terminal of the transistor 12 is coupled to an output terminal of an inverter 14. An input terminal of the inverter 14 receives a control signal SN. The transistor 11 is turned off by the transistor 12 in response to the disabling state of the control signal SN. The transistor 12 is controlled by the control signal SN through the inverter 14. The transistor 12 is turned on to switch off the transistor 11 in response to the disabling state of the control signal SN. However, when the transistor 12 is turned on, the resistor 15 will consume a power PR, it is given by,
                              P          R                =                              V            IN            2                                R            15                                              (        1        )            wherein R15 is a resistance of the resistor 15. The voltage source VIN is normally supplied from an AC power source. Through the rectification, the voltage of the voltage source VIN would be as high as 350VDC when a high line voltage is applied. Therefore, a significant power loss will be produced at the resistor 15. The resistor 15 in high resistance such as several mega ohms can be used to reduce the power loss. However such the resistor 15 in high resistance is not appropriate to be built into an integrated circuit. Therefore, it is desirable to develop a high efficiency start up circuit, especially to develop an integrated circuit for high voltage start up.